What happen if I go underwater without Helmet of the Deep ?

Question

One thing I always want to know, what will happen if I go underwater withtout the Helmet or take off the Helmet while underwater ?

Answer 1

Helmet is required to be able to go underwater, but once there the helmet can be removed.  However, without it players will take 5 times 20 Drowning damage kicking in after four seconds of standing still. It is useful out of the depths, too: some monsters, like Water Elementals use drowning damage attacks.

That means you won't be able to go underwater if you not wearing it :)

Source : TibiaWiki

Answer 2

In addition to  aneg's answer, if you stay un the same sqm underwater without moving, you won't lose HP by drowning after those 5 turns of 20 drowning damage, even if you are not wearing the proper helmet. Only when moving you lose It again.